The invention relates to an easy to open package wherein an article is sealed between upper and lower thermoplastic webs, and to a method of making such package. More specifically the invention is directed to a package which is tightly sealed and will not be easily destroyed by the usual handling during shipment and storage but which may easily be opened by manually peeling apart the upper from the lower web of the package.
A wide variety of products, especially food products like meat, sausages, cheese and the like are being offered in visually attractive packages made from two thermoplastic webs using the vacuum skin packaging, thermoforming or other processes. In a vacuum skin packaging process a product is placed on a support film or formed tray of relatively rigid material and, by differential air pressure, a heated film above the product is molded down upon and around the product and against the support, the space between the upper film and the support having been evacuated. The upper heated film forms a tight skin around the product and is sealed to the support. In the thermoforming process the upper thermoplastic web is sealed to the flange-like edges of the support using heated sealing bars or similar equipment.
The problem that is encountered is that the heated film seals so strongly to the support that it is difficult to separate the two webs and to open the package. An easy to open package comprising two thermoplastic webs safely sealed together should be easily openable by manually pulling apart the two webs, normally starting from a point like a corner of the package where the upper web has purposely not been sealed to the support. In this manner the use of scissors, a knife or other devices to open the package can be avoided.
The easy to open packages of the prior art are based on two different mechanisms. The first concept is being widely used in thermoformed, formfill-seal and also in vacuum skin packages. When opening the package the separation between the two thermoplastic webs occurs at the seal interface, i.e. between the two contact surfaces sealed to each other. The opening or peeling strength (force per given width) is therefore identical to the seal strength and may be controlled by appropriate choice of the chemical similarity or dissimilarity of the two sealed layers.
It is a disadvantage of this type of easy opening mechanism that it is strongly effected by the conditions of the sealing process. Thus, too high sealing temperatures will result in a stronger seal so that in turn the package can no longer be easily opened. Further, the opening or peeling strength will widely differ for a series of packages and a rupture of the upper web may occur during opening when it is sealed too strongly to the support web.
The second and more recent easy opening mechanism is currently being used in many applications, for instance for rigid thermoformed containers sealed with a flexible film where the sealed areas are relatively narrow. Basically the opening of the package is achieved through an initial breakage through the thickness of one of the sealing layers followed by delamination of this layer from the underlying support or film. A typical example is a system where both the first and the second seal layer are made from polyethylene and one of the seal layers is adhered to a polyamide surface. The low bond strength between the polyethylene and the polyamide permits the delamination to take place during opening of the package. When the delamination reaches the area of the packed product a second breakage through the sealing layer takes place. As a result the entire sealing layer of one of the two webs is separated from one of the webs and is left sealed to the opposite web.
The opening strength is directly related to the bond strength between the seal layer and the adjacent film layer and accordingly it depends primarily on the chemical similarity or dissimilarity of the two materials. However, the coextrusion conditions such as pressure, temperature and time of contact between the molten materials have also a major effect on the final bond strength between the two layers.
The above described prior art easy opening systems are all based on an "adhesive failure" that takes place either at the seal interface between the two seal layers or through delamination of the seal layer from the corresponding underlying web. The systems are characterized by an opening strength which is not constant and can be either too low (which may lead to an inadvertent opening of the package) or too high (making it difficult to manually open the package). The opening strength of the known systems is significantly affected by the production conditions, the packaging machine conditions, and the environmental conditions (mainly temperature) during the packaging process and storage.